1. Field of the Invention
The invention relates to a system for detecting the position of a device termed AGM, and the time that a smart pig moving through a pipeline passes the position of the AGM.
2. Background of the Invention
An Above Ground Marker (AGM) is typically used in pipeline applications involving the use of a device that passes through the pipeline to collect various data with respect to the condition of the pipeline, such as corrosion, deformation and the like. These devices are commonly termed a xe2x80x9csmart pigxe2x80x9d.
A conventional AGM is placed on top of a pipeline and detects the time when a pig passes by the AGM location in the pipeline. The AGM typically has an internal logic system to detect and record the time of the pig pass. The position of the AGM, however, must be determined by a global positioning system (GPS). This conventional practice has two significant disadvantages. The first one is that two separate crews are required. One crew must determine the position of the AGM and the other the time of the pig passage.
A GPS receiver produces two types of measurements, one termed xe2x80x9cpseudo-rangexe2x80x9d and another termed xe2x80x9ccarrier phasexe2x80x9d. These measurements provide information regarding the distance from a satellite to the receiver antenna. These measurements, however, contain errors caused by block biases in the satellite and receiver, atmospheric delays, and multi-path effects. With pseudo-range measurements the absolute distance is determined but a noise level of several meters is present. The carrier phase measurement has a noise level of only a few millimeters but provides only relative distance measurement. There are basically three techniques for using these GPS measurements that are single point xe2x80x9cstand-alonexe2x80x9d positioning, differential positioning xe2x80x9cDGPSxe2x80x9d and kinematic positioning xe2x80x9cgeodetic surveyingxe2x80x9d.
With single point positioning, only one receiver is employed. This receiver provides only pseudo-range measurements. With this practice, with a minimum of four satellites being visible, the position of the receiver can be computed by triangulation. Imperfect modeling is employed to remove system propagation errors, but the removal is incomplete. The resulting residual range error and the random noise of the measurement results in a final positioning accuracy of about 10 meters horizontally and 15 meters vertically. This accuracy is not sufficient for effective detection of a position of a smart pig moving through a pipeline.
To enhance the position accuracy, differential positioning xe2x80x9cDGPSxe2x80x9d is employed. With this technique, a receiver termed xe2x80x9cbasexe2x80x9d occupies a known location so that the actual errors in each range measurement can be accurately computed. This range correction is applied to each xe2x80x9croverxe2x80x9d receiver range measurement to remove the error components that are common to both the base receiver station and the rover receiver station. With this practice, only geometrical information is provided by the rover receiver measurements regarding its position so that the final triangulation is free of errors that are common to both receivers. Although this practice can provide position error of meter to sub-meter levels, it is nevertheless not sufficient for the required position detecting of a smart pig moving through a pipeline.
With kinematic positioning the GPS uses a carrier phase measurement. This constitutes a continuous count of carrier waves beginning with the first signal lock. As a receiver/satellite range measurement it provides only relative changes occurring after the first lock. Consequently, the initial integer count, termed xe2x80x9cinitial ambiguityxe2x80x9d, is unknown. Consequently, to effectively use the carrier phase measurement, the initial ambiguity must be resolved. To remove this and other error components contained in the pseudo-range, double differencing is used. With this procedure, carrier phase measurements are first differenced between two receivers, and then between satellites for the rover receiver. These measurements are free of errors, such as satellite and receiver clock biases, satellite orbit errors, ionospheric and thropospheric delays. Once these double differenced initial ambiguities are resolved, the double differenced carrier phase measurements will provide type centimeter-level positioning results. Unlike single point and differential positioning techniques, kinematic positioning requires post-mission processing, and thus the use of two crews. This results because the resolution of the initial ambiguities requires further processing and a determination of the geometry change of the satellite. This practice requires an extended time interval.
With the two-crew practice, one AGM crew installs the AGM devices to record time of pig passage and another GPS crew is required to survey the AGM locations by recording raw GPS measurements at each location. These recorded GPS measurements are then processed as discussed above to achieve centimeter-level accuracy.
With the practice of the invention these two processes are combined by using a GPS receiver at the AGM location, and recording raw GPS measurements at each location. In addition, a remote notification function may be included at the AGM location. This may constitute a cellular telephone or a satellite link to a remote monitoring site that is activated by the AGM when the pig passes the AGM location to broadcast in real time the time of passage of the pig and AGM position. Hence, only one crew is required to achieve both high accuracy positioning and pig passage time.
It is accordingly an.object of the present invention to provide for the time and position marking by a single apparatus so that the position of the AGM only has to be occupied once, with only a single crew being required.
In accordance with the invention there is provided an apparatus for detecting the position of a device, such as a smart pig, moving through a pipeline. The apparatus includes means positioned at a first location above the pipeline for producing a first electrical signal in response to passage of the device through the pipeline at the first location and a second signal to activate a first global positioning system positioned at said first location to provide a third electrical signal indicating the position of said first location and a second global positioning system positioned at a second selected, known location remote from said first location to provide a fourth electrical signal to correct said third electrical signal.
The means for producing the first electrical signal may further include means for collecting and storing this first electrical signal.
The first electrical signal recording may be activated by the passage of the device through the pipeline at the first location.
The apparatus may further include a computer for collecting said first electrical signal, for collecting and carrier phased based double differencing said third and fourth electrical signals from the global positioning systems to provide a position signal of the first location and further for integrating the first electrical signal indicative of the time that the device passes the first location and the position signal to provide an indication of the time of the device passing this position of the first location along the pipeline.
The apparatus may comprise an above ground marker, positioned at a first location above the pipeline, for producing, collecting and storing a first electrical signal in response to and indicative of a time of passage of the device through the pipeline at the first location and a second signal to activate a first global positioning system positioned at the first location. A second global positioning system may be positioned at a selected, known location remote from the first location. The first and second global positioning systems may be each adapted for producing first and second electrical signals, respectively, indicating the respective positions of each of the global positioning systems. A computer may be used for collecting the first signal and carrier phased based double differencing the first and second signals from the global positioning systems to provide a position signal of the first location and for integrating the first electrical signal indicative of time of passage at the first location and the position signal to provide an indication of the time of the device passing the position of the first location. Means may be provided at the first location for real time broadcasting of time of passage of the device at this location and the position of this location to a remote monitoring site. This means may be activated by the AGM.